This invention relates generally to backfill materials for ground bed anodes, particularly of the vertically positioned type, and to improvements in vertically positioned anode beds, particularly deep beds of the general type described in U.S. Pat. No. 3,725,669 to Joe F. Tatum.
The practice of utilizing deep well anode beds to prevent corrosion and rapid deterioration of subsurface metallic structures is an effective method of increasing the life of such structures.
Under various conditions, corrosion of subsurface metallic structures is electrolytically induced by the creation of anodic and cathodic areas on the metallic structures. It was found that corrosion occurred at the anodic area of the structure at which a current flow was established into the surrounding soil and water which acted as an electrolytic medium. However, the cathodic areas of the subsurface structures at which the flow of current was directed or collected from the surrounding medium were found to remain relatively free from corrosive action.
In order to prevent the corrosive action at anodic areas along the subsurface structures, it was determined that various forms of electrodes could be placed in the ground adjacent to the structure and a current supplied thereto and into the surrounding soil to the structure. In this manner, the electrode acted as an anode which became subject to electochemical attack and the subsurface metallic structure was protected from such corrosive action as its surface was established as a cathodic area by connecting conductors between such structure and the current source. Such a system has become known in the field as cathodic protection.
Cathodic protection has been widely accepted. However, its effectiveness depends on the effective life of the electrode used to establish current flow. Early electrodes consisted of utilizing metallic pipes, rails, beams and various metal scraps which were buried in the ground adjacent the subsurface structure to be protected. Since such electrodes were subject to corrosive effects, their maximum effective life was dependent upon the weight of the material, the amount of current used, and the soil conditions including soil acidity and moisture content.
In use such electrodes tended to separate along areas of localized corrosion and therefore portions of the electrode were removed or separated from the current supply. Such localized corrosion substantially decreased the effective life of the electrode resulting in an effective life range generally between four to eight years, depending upon the various conditions mentioned above.
In order to provide continuous cathodic protection it is necessary to replace the expended electrodes, adding significantly to the expense of maintaining such a system. In addition to the metallic anodes previous described, various carbon and graphite electrodes have come into widespread use.
From the above, it is apparent that in order to increase the economical operation of a cathodic protection system, it is desirable to utilize electrodes having a low rate of consumption in terms of pounds of electrode per ampere per year. Further, the cost of electrode replacement is an important consideration.
As discussed above, the rate of consumption of the anode material was subject to various factors including possible localized separation. In this respect, it was noted that the rate of consumption was dependent upon the current density at the interface of the anode and the soil medium. In order to provide or establish a more uniform flow of current along the length of the anode, use was made of a uniformly resistive backfill material to completely surround the anode. Material including granular, fine grain or pulverized carbon substances including calcined coke, graphite and the like became frequently used not only to provide a uniformly resistive medium but also to effectively decrease the electrical resistance of the circuit material between the anode and the protected material cathode. As discussed in U.S. Pat. No. 2,553,654 to Heise, the use of such backfill permitted a significantly increased current density along the anode.
The backfill has customarily been poured around the anode or anodes and permeated with water in order to promote electrical conductivity between the anode system and the earthen wall of the well. A deep well system employing backfill is described in the above mentioned patent to Tatum U.S. Pat. No. 3,725,669.
Inasmuch as any well may intersect with water bearing strata at different levels it is apparent that a well for anodes for the purpose of providing cathodic protection, and having water permeable backfill between the strata, may permit the transfer of liquid to and between the strata. With greater attention now being directed to the preservation of water quality it becomes desirable or necessary to construct anode beds in wells in such a manner as to prevent the transfer of liquid from the well hole to any water bearing stratum. Such liquid may originate at the surface and flow downwardly into the well or it may pass from any stratum into the well and then into another stratum. The passage of liquid from outside a given stratum into it may be inconsistent with efforts to prevent contamination of the environment.